1. Field of the Invention
The invention relates to an apparatus for heating sealing two superposed layers of plastic film.
2. Description of the Prior Art
In a plastic bag making machine, an apparatus has been generally used for heating sealing two superposed layers of plastic film, as disclosed in Japanese Laid-Open Patent Publication No. 197,119 of 1991. The apparatus includes a plurality of fixed sealing bars and a plurality of movable sealing bars disposed opposite to the fixed sealing bars respectively. The plastic film is folded in two superposed layers, the layers of plastic film being passed between the movable and fixed sealing bars. Feed rollers are engaged with the layers of plastic film and rotated by a servo motor so that the layers of plastic film are intermittently fed for a length and temporarily stopped whenever intermittently fed. A rocker is operatively connected to the movable sealing bars. A crank is operatively connected to the rocker and rotated by a servo motor. The rocker is rocked by the crank so that the movable sealing bars are reciprocatingly moved by the rocker, between first and second positions. In the first position, the movable sealing bars are advanced toward the fixed sealing bars so that the layers of plastic film are sandwiched between the movable and fixed sealing bars. In the second position, the movable sealing bars are retracted and spaced from the fixed sealing bars and the layers of plastic film. The layers of plastic film are sandwiched between and heat sealed by the movable and fixed sealing bars when temporarily stopped.
In the apparatus disclosed in the above Japanese Laid-Open Patent Publication, the feeding time and the heat sealing time for the layers of plastic film are continuous with each other so that the layers of plastic film are sandwiched between and heat sealed by the movable and fixed sealing bars, as soon as temporarily stopped. The layers of plastic film are then fed again as soon as the movable sealing bars are retracted from the layers of plastic film. The cycle time of the apparatus is therefore determined by the plastic film feeding and heat sealing times, plastic bags are being made at every cycle time. Accordingly, the apparatus has a problem that when the plastic film feeding and heat sealing times are once selected, the making speed of plastic bag can not be optionally changed. This is inconvenient. For example, when the layers of plastic film are experimentally fed and heat sealed for adjusting the apparatus after the selection of the plastic film feeding and heat sealing times, the plastic film feeding and heat sealing times are required to maintain at selected values. In contrast, the plastic bag making speed is not required to maintain at selected value. It is desired from the viewpoint of plastic film material cost to lower the plastic bag making speed. However, when the plastic film feeding and heat sealing times are once selected, the cycle time of the aparatus and the plastic bag making speed are determined by the selected values so that the plastic bag can not be lowered.